A rack and pinion steering gear unit is used as a mechanism for converting rotating motion that is inputted from a steering wheel to linear motion for applying a steering angle to steered wheels. Construction of a steering apparatus that comprises this kind of rack and pinion steering gear unit is already widely used as disclosed in JP 2009-184591 (A), JP 2004-17872(A) and JP 61-124471(U).
FIG. 6 illustrates an example of a conventional steering apparatus in which a rack and pinion steering gear unit is assembled. In this steering apparatus, in order to reduce the operating force of the steering wheel 1, an auxiliary operating force from a motor 3 that is installed in the middle section of the steering column 2 is applied to the steering shaft (not illustrated in the figure). The movement of the steering shaft that rotates as the steering wheel 1 is operated, is transmitted to an intermediate shaft 4, and by way of a pinion shaft 5, causes the rack shaft 9 (see FIG. 7) of a rack and pinion type steering gear unit 6 to move back-and-forth, which steers the steered wheels by way of a pair of left and right tie rods 7.
As illustrated in FIG. 7 and FIG. 8, in a rack and pinion steering gear unit 6 of this kind of steering apparatus, pinion teeth 8 that are formed on part in the axial direction (tip end half section) of the pinion shaft 5 engage with rack teeth 10 that are formed on the front surface (surface on the top side in FIG. 7 and FIG. 8) of the rack shaft 9. A helical rack having a twisted angle is used for the rack teeth 10, and to correspond to this, a helical gear having a twisted angle is used for the pinion teeth 8. Part of both the pinion shaft 5 and the rack shaft 9 are housed inside a housing 11.
The housing 11, which is a member of the steering gear unit 6, comprises a main housing section 12 and a sub housing section 13, which are both cylindrical in shape. The main housing section 12 is open on both ends, and the sub housing section 13 is provided on the side of part of the main housing section 12, and one end is open. The center axis of the main housing section 12 and the center axis of the sub housing section 13 are in a twisted position against one another.
The rack shaft 9 is inserted through the main housing section 12 so as to be able to displace in the axial direction, and both end sections thereof protrude from the main housing section 12. The tip end half of pinion shaft 5 where the pinion teeth 8 are formed is supported on the inside of the sub housing section 13 so that only rotation is possible. In order for this, the other end of the pinion shaft 5 (left end in FIG. 8) is supported by the back end section of the sub housing section 13 by way of a radial needle bearing 14. Moreover, the middle section of the pinion shaft 5 is supported by a deep-groove, a three point contact or a four point contact single-row ball bearing 15 in the portion near the opening of the sub housing section 13. Of the inner ring 16 and the outer ring 17 of the ball bearing 15, the inner ring 16 is held between an inner-diameter side stepped surface 18 that is formed in the middle section of the pinion shaft 5 and a conical shape snap ring 19 that is fastened to the middle section of the pinion shaft 5. Moreover, the outer ring 17 is held between an outer-diameter side stepped surface 20 that is formed in the middle section of the inner circumferential surface of the sub housing section 13 and a locking screw cylinder 21 that is screwed onto the opening section of the sub housing section 13. With this construction, the tip end half section of the pinion shaft 5 is supported inside the sub housing section 13 so that radial loads and thrust loads can be supported, and so that displacement in the axial direction is prevent and only rotation is possible. In this way, it is possible to maintain the engaging section between the rack teeth and the pinion teeth in the proper state of engagement, and it is possible to prevent the occurrence of strange noise in the engaging section.
During operation of this kind of rack and pinion steering gear unit 6, a force is applied to the rack shaft 9 in a direction away from the pinion shaft 5 due to a reaction force that occurs in the engaging section between the pinion teeth 8 and the rack teeth 10. Therefore, a mechanism for supporting the rear surface side of the rack shaft 9 is provided on the opposite side from the engaging section between the pinion teeth 8 and the rack teeth 10, which prevents displacement of the rack shaft 9 in a direction away from the pinion shaft 5.
As this kind of mechanism, a cylinder section 22 is provided in the housing 11 in the portion on the opposite side of the sub housing section 13 in the diametrical direction of the main housing section 12, and a slide type rack guide 23 is provided inside this cylinder section 22. This slide type rack guide 23 is mounted inside the cylinder section 22, and comprises a pressure block 24 that has a partial cylindrical concave surface that corresponds to the shape of the back surface of the rack shaft 9 on side that presses the rack shaft 9, a cover 25 that is screwed onto the opening section of the cylinder section 22, and screw threads that are formed between the pressure block 24 and the cover 25, and the pressure block 24 is constructed so as to press toward the rack shaft 9.
With this construction, together with eliminating backlash in the engaging section between the pinion teeth 8 and the rack teeth 10, it is possible to maintain the engaging section between the pinion teeth 8 and the rack teeth 10 is a proper state of engagement regardless of the force that is applied to the rack shaft in a direction away from the pinion shaft 5 as power is transmitted at the engaging section between the pinion teeth 8 and the rack teeth 10.
In the case of this kind of steering apparatus, in order to support the tip end half section of the pinion shaft 5 of the rack and pinion gear unit 6 inside the sub housing section 13 so that radial loads and thrust loads can be supported, the locking screw cylinder 21 is screwed onto the opening section of the sub housing section 13, which regulates the displacement on the axial direction of the outer ring 17 of the ball bearing 15. Therefore, it is possible to increase rigidity against thrust loads that try to cause the pinion shaft 5 to displace in the axial direction. However, by simply providing this locking screw cylinder 21, the dimension in the axial direction of the housing 11 increases, so there is a possibility that the size of the steering gear unit 6 will increase. Moreover, the work of assembling this locking screw cylinder 21 is troublesome, and there is a possibility that the production cost will increase.
JP 2010-38254 (A) discloses construction of a rack and pinion gear unit that uses a tapered snap ring such as illustrated in FIG. 9 instead of the locking screw cylinder 21 having the construction illustrated in FIG. 8. In this rack and pinion gear unit 6a, a plurality of conical shaped protrusions 46, which protrude from the inclined side surface 30 that is formed in the portion near the outside end in the radial direction of one side surface in the axial direction of the tapered snap ring 28, are formed at evenly space positions in the circumferential direction of that inclined side surface 30. Moreover, a ring shaped concave groove 47 is formed in an inclined groove side surface 32 of one side surface in the axial direction of the inside surfaces of a ring shaped locking groove 27 that is formed around the inner circumferential surface of the housing 11 for locking the tapered snap ring 28. In the case of this kind of construction, when the tapered snap ring 28 contracts a specified amount due to the radial component of a reaction force from the inclined groove side surface 32 of the locking groove 27 in reaction to an outward force (thrust load) in the axial direction that is applied to the tapered snap ring 28, the protrusions 46 of the tapered snap ring 28 engage with the inner wall surface (inside surface in the radial direction) of the ring shaped concave groove 47. In this way, by being able to prevent the tapered snap ring 28 from contracting more than a specified amount, and stabilizing the locked state between the tapered snap ring 28 and locking groove 27, movement in the axial direction of the tapered snap ring 28 is regulated. With this kind construction, the dimension in the axial direction of the housing 11 does not increase, and the work of assembling and locking the tapered snap ring 28 in the locking groove 27 is not troublesome. However, there is a problem in that the shape of the tapered snap ring 28 and the locking groove 27 that is formed around the inner circumferential surface of the housing 11 is complicated, so there is processing is difficult and thus the manufacturing cost increases.